Respect phpstan level in InvalidKeyInArrayDimFetchRule

[VincentLanglet]

Extracted from #4012

It will be easier to review the CastArrayKeyRule after

[ondrejmirtes]

If you're doing something to respect levels better, you need to add some cases to LevelsIntegrationTest. You can just add the "topic" php file and run the test, it will generate JSONs with the current result.

It'd actually be great to see the current behaviour in the first commit and then see how things get corrected in the second commit with the code changes.

The expectations are:

• level < 7 - report things that are always wrong
• level 7 - report things that might be wrong, like int|string only where int is expected
• level 8 - same but for int|null
• level 9 - explicit mixed
• level 10 - implicit (untyped) mixed

[ondrejmirtes]

The best way to get LevelsIntegrationTest results fast is to comment out all the other "topics" when you're working on a single topic. Otherwise it takes a long time for the tests to run.

Also you could add some upload-artifact action call to tests-levels in tests.yml so that when PR run fails, we can download the updated JSON files and there's no need to run the test locally and slowly.

[VincentLanglet]

Ok, so here you have

• First commit introduce the integration test with the current behavior
  5e15e2e

• Second commit is my fix of InvalidKeyInArrayDimFetchRule and the impact
  ae06b48

• Funny things, the message is different between level 7 and 8, and I found the fix needed was in RuleLevelHelper:
  38c316f
  -> You should not remove null if it's an accepted type in the Union (because if you do so, you might transform a "maybe error" into a "100% sure error".

Then last commit was just to fix the lint 7.4.